A ModelMap implementation of RedirectAttributes that formats
values as Strings using a DataBinder. Also provides a place to store
flash attributes so they can survive a redirect without the need to be
embedded in the redirect URL.

addAttribute

Note: Empty Collections are not added to
the model when using this method because we cannot correctly determine
the true convention name. View code should check for null rather
than for empty collections as is already done by JSTL tags.